A Houthi Shiite fighter stands guard as people search for survivors under the rubble of houses destroyed by Saudi airstrikes near Sanaa Airport, Yemen. Saudi Arabia launched airstrikes March 26 targeting military installations in Yemen held by Shiite rebels who were taking over a key port city in the country's south and had driven the embattled president to flee by sea, security officials said.

Washington — With another civil war looming in the Middle East in Yemen, President Obama has been served another sharp reminder that reducing the weight of the Middle East in America’s national security portfolio is more easily envisioned than accomplished.

Not long ago it was the Syrian war that exploded and raised the prospect of renewed US military intervention in the region. But Mr. Obama was determined not to see America dragged into another Mideast war – and he fashioned a Syria policy accordingly.

The Syrian civil war is entering its fifth year, with the US remaining largely on the sidelines. But in the eyes of many critics at least, the lack of US intervention gave rise to the self-proclaimed Islamic State, the extremist Sunni Islamist group spreading its tentacles beyond Syria and Iraq to Libya and deeper into Africa.

Now fighting in Yemen between a government–on-the-run and Sunni tribesmen supported by Saudi Arabia on one side, and Iran-backed Shiite Houthi rebels on the other, threatens to turn into a proxy war between Iran and Saudi Arabia.

Left to intensify, the fighting could degenerate into a new front in the region’s Sunni-Shiite schism and a wider regional war that would be increasingly difficult – and dangerous – for the US to sit out.

Two factors underscore why, and help explain why Obama’s vision of “rebalancing” US interests and foreign policy toward Asia is likely to fade further with war in Yemen.

Yemen’s strategic position along the Gulf of Aiden, one of the world’s busiest shipping lanes, and at the southern mouth of the Red Sea, an important petroleum-transport corridor, is one factor.

But the other is perhaps even more disconcerting for American national security interests: Yemen is home to Al Qaeda in the Arabian Peninsula (AQAP), which is considered by terrorism experts to be Al Qaeda’s most active and threatening brand, particularly in terms of its ability to mount terrorist attacks against the West.

A protracted war that leaves Yemen a failed state would not just pose a threat to the neighbors, including Saudi Arabia, which shares a 1,100-mile-long border with Yemen, but also to the US.

Until the advancing Houthis forced US-backed President Abed Rabbo Mansour Hadi to flee the capital of Sanaa in late February (and then the country last week), the US had special forces in Yemen to assist in the fight against AQAP. Now the US has pulled those forces, along with all of its diplomats. The chaos of an extended Houthi-Sunni fight in Yemen could work to AQAP’s advantage.

Obama has signaled that he plans to employ the lead-from-behind strategy toward the Yemen fighting that he followed in the fight against former dictator Muammar Qaddafi in Libya in 2011. In that effort, the US took a backseat as Europeans led the air strike campaign against the Qaddafi regime.

Last week, Obama authorized logistical and intelligence support for the Saudis in their campaign of air strikes against advancing Houthi forces. Plans to implement a strictly supportive role were evident in administration statements.

“While US forces are not taking direct military action in Yemen in support of this effort, we are establishing a Joint Planning cell with Saudi Arabia to coordinate US military and intelligence support,” National Security Council spokeswoman Bernadette Meehan said Wednesday.

At the same time, the US announced its support of a newly formed coalition of 10 Sunni Arab countries designed to back the Saudi-led bombing campaign against the Houthis. Egypt, part of the group, has floated the idea of sending in ground troops to thwart the Houthis – who besides being Shiites are also Arabs.

For many critics of Obama’s stand-back approach to the Middle East’s mounting conflicts, the most apt description for the strategy is “incoherent.” Exhibit A for their criticism is the juxtaposition of the situation in Iraq with that of Yemen.

In Iraq, US forces are bombing IS positions in Tikrit in support of Iran-backed and -commanded Shiite militias on the ground; while in Yemen, the US is providing logistical support to the Saudis, who are bombing Iran-backed rebel forces.

The administration’s response appears to be that the US is not acting to tip the balance one way or the other in the region’s centuries-old Sunni-Shiite schism, but is acting in the interest of US national security and in support of regional stability.

Thus in Iraq, the US finds itself on the same side as Iran is it assists the Iraqi government, through military training and advising and now air strikes, in its efforts to dislodge IS forces from Iraqi cities; and in Yemen, the US is on the opposite side from Iran as it assists the Saudis and other Sunni Arabs seeking to repel Iran-backed insurgents.

The administration’s more long-term goal appears to be to hand off responsibility for the region’s security (or at least a greater share of it) to a set of more or less balanced regional powers – including Iran, to the horror of longtime American allies Saudi Arabia and Israel.

One reason Obama backed off getting the US involved militarily in the Syrian conflict in September 2013, despite the Syrian president crossing the White House’s red line on using chemical weapons, to avoid making the Middle East the focus of the rest of his presidency and killing any chance of implementing a supporting-role-only Mideast policy.

Now the Yemen conflict threatens, to the president’s chagrin, to ensure that Obama leaves office with the US still mired in conflict in the Middle East.